Electron discharge device



May 23, 1939. J. H. DE BOER I ELECTRON DISCHARGE DEVICE Filed April 8, 1957 INVENTOR JAN HENDRIK DE 505/? ATTORNEY Patented May 23, 1939 UNITED STATES PATET oFFicE assigncr to rieken N. V. Philips Gloeilampenfab- Application April 8, 1937, Serial No. 135,623 In the Netherlands April 14, 1936 3 Claims.

My invention relates to improvements in cathode ray tubes, and particularly to improvements in tubes utilizing fluorescent screens which contain sulphur or sulphides.

5 Conventional cathode ray tubes comprise an evacuated envelope enclosing a cathode and an electrode structure forming an electron gun for generating and projecting a high velocity beam of electrons on a fluorescent screen. Fluorescent l screens for producing white light have been made of screen materials, generally mixtures containing zinc sulphide or zinc cadmium sulphide. It has been found that such tubes have an unsatisfactory life, and that under normal operating 15 conditions the cathode appears to deteriorate in such a manner that after a relatively short time the electron emission from the cathode is considerably reduced.

It is an object of my invention to provide a 20 cathode ray tube having a fluorescent screen containing sulphur or sulphur compounds, such as sulphides, in which the cathode maintains its electron emitting characteristics substantially unchanged during the useful life of the device.

It is a further object of my invention to provide such a cathode ray tube with means for minimizing deterioration of the cathode during use.

In accordance with my invention, a cathode 30 ray tube having an electron gun and a fluorescent screen of materials containing sulphur or sulphides is provided with means adjacent the screen for neutralizing or eliminating the deleterious effects of free sulphur or other decomposition prod- 35 ucts released from said screen. To this end I provide on a portion of the envelope between the electron gun and the screen a film or coating consisting predominantly of alkaline earth metal, preferably as a band or ring as close to the 40 screen as feasible.

Other objects, features, and advantages of my invention will appear from the following description taken in connection with the drawing in which the figure is a longitudinal view of one 45 form of cathode ray tube incorporating my invention.

Referring tothe figure, the tube comprises a highly evacuated frusto-conical envelope I having an elongated cylindrical neck section 2. The

50 frusto-conical section of the envelope has a substantially fiat transparent end '3 having on its inner surface a coating of fluorescent material 4 containing a sulphide of a'metal, such as zinc, cadmium, or other metals commonly used. The

55 neck section 2 encloses the electron beam gen- .ably comprises a metal base with a coating of 5 barium and strontium oxides 6 which when heated produces a concentrated stream of electrons which may be focused and projected, as an intense electron beam on the fluorescent screen 4 by the anodes l and 8. 10

In accordance with my invention I deposit an active metal, such as barium or other alkaline earth metal, as a thin film and preferably in the form of a relatively Wide band or ring9 on the inner surface of the envelope between the anode 8 and the fluorescent screen 4, and preferably closer to the screen than to the anode. The deposition of the metal may most conveniently be accomplished by vaporizing the barium or other active metal, for example from pellets of a ,20 barium-aluminum alloy supported in a plurality of apertured metallic cups H located at intervals adjacent that portion of the wall to be coated. The vaporization of the active metal, such as barium, may then be accomplished by heating the individual cups containing the metal to be vaporized by subjecting them to a high frequency field thereby heating them to the vaporizing temperature of the metal in a manner well known in the art.

Cathode ray tubes having fluorescent screens containing sulphides have shown considerably longer and more satisfactory life when made in accordance with my invention than when made in the conventional way. While I do not desire to be limited to any particular theory of operation, it seems probable that this improvement is due to the binding or taking up of sulphur or other decomposition gases or products from the fluorescent screen by the film of barium or other active metal adjacent the screen, with consequent reduction to a minimum of positive ion bombardment of the cathode. Probably the film of active metal is at a negative potential, since the glass walls of the envelope are known to become negative during operation of the tube, and such a potential tends to enhance the effect of the film.

From the foregoing description it will be apparent that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

1. In combination, a cathode ray tube having an evacuated envelope, an oxide coated cathode adjacent one end of and exposed to the residual atmosphere in said envelope, and a luminescent screen oppositely disposed from said cathode containing a compound of sulphur capable of forming deleterious decomposition products injurious to the oxide coating of said cathode, a coating of an active metal capable of binding deleterious gases formed by said screen positioned on the wall of said envelope between said anode and said fluorescent screen out of contact with and removed from said luminescent screen.

2. In combination, a cathode ray tube including a highly evacuated envelope, an oxide coated cathode subject to injury by deleterious gases formed in said envelope and a luminescent screen of a compound of sulphur which is inherently capable of forming such deleterious gases, a coating of alkaline earth metal to absorb the deleterious gases formed by said screen in the form of a ring on the wall of said envelope between said cathode and said screen, adjacent said screen and out of contact with said screen.

3. In combination, a cathode ray tube having an evacuated envelope, an oxide coated cathode subject to injury by deleterious gases adjacent one end of said envelope and an oppositely disposed luminescent screen including a sulphide of a metal selected from the group consisting of zinc and cadmium which is inherently capable of forming such gases deleterious to the said oxide coated cathode, a coating predominantly of barium on the wall of said envelope between said cathode and said screen, adjacent to, out of contact with, and separated from said screen.

JAN HENDRIK DE BOER. 

